[postgis-users] bow-tie polygons and the like

[Chris Hermansen]

Good people;

I have this odd problem that I hope others may clarify for me.

I have a big ArcInfo polygon coverage produced by a sequence of spatial
unions.  One of the component polygon coverages was brutalized somewhere
along the way and it has some polygons with triangular and bow-tie
inclusions.

Nevertheless, the coverage is clean and sober as far as ArcInfo is
concerned.

If you all can picture in your minds a bow-tie polygon |><| where the
rightmost | of the bowtie is actually the edge of an enclosing polygon,
kind of like:

      |
|><|
      |

(I sure hope that works out visually...)

OK, when I use ogr2ogr to import this coverage into postgis 1.3.1 /
geos-3.0.0rc4, I see the following:

    * the containing polygon fails at st_isValid()
    * the st_area() of the containing polygon appears to include the
      area of the bow-tie polygon, ie it is larger than the area copied
      over from the ArcInfo coverage by about the same amount as the
      area of the bow-tie polygon

When I look at this little mess with OpenJump, sure enough the QA
routines find the bow-tie and complain.

The long and the short of this is that PostGIS returns slightly more
area than ArcInfo does for the same big ugly polygon coverage, because
of the apparent loss of these bow ties.

What I'm not sure about is what exactly is wrong here.  Is the bow-tie
shape itself illegal?  Or is it the fact that both sides of the bow-tie
belong to the same polygon?  Or something else I'm missing?

Thanks for any light anyone can shed on this.

-- 
Regards,

Chris Hermansen · mailto:clh at timberline.ca
tel:+1.604.714.2878 · fax:+1.604.733.0631
Timberline Natural Resource Group · http://www.timberline.ca
401 · 958 West 8th Avenue · Vancouver BC · Canada · V5Z 1E5

C'est ma façon de parler.

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